The present invention relates to a method and apparatus for generating a synchronizing signal for a firing-circuit subassembly to trigger a power converter valve of a controlled series compensator.
In series compensation, capacitors are usually used in the line section in order to gradually reduce the transmission phase angle and the current-related voltage drop on the line. These capacitors are capacitor banks that are connected to and disconnected from the system either as a group or as a plurality of subcapacitors (segments) in series. The capacitor is connected and disconnected by opening or closing a parallel circuit breaker. Protection for the capacitor, in the event of a network short circuit, is provided by a parallel shunt, a triggerable spark gap, and/or a parallel circuit breaker.
Also in a series compensation system, the total impedance of the series compensator (by analogy with the TCR [thyristor controlled reactor] in a static compensator) is steplessly controlled by means of an inductor wired in parallel with the capacitor with a power converter valve at high voltage by firing in an appropriate manner. A series compensator controlled in this manner is known as an ASC (advanced series compensator). With a series compensator controlled in this manner, the dynamics of series compensation can be improved and the total impedance can be controlled within a certain range. The impedance can also be changed from capacitative to inductive.
Series compensators of this kind are described in the article "Regulated Parallel and Series Compensators," printed in the German journal "Elektrie," volume 45, March 1991, pages 88-90. A series compensator of this type, that is integrated into a transmission line, is also described in the international patent document WO 87/04538.
The operation of the controlled series compensation system is based on the triggering of a power converter valve that generates, by means of a specific charge-reversal process through the coil, an impedance affecting the power network. This triggering must repeat periodically, and is related to the voltage through the capacitor of the controlled series compensation system. A firing-circuit subassembly ensures time-synchronized triggering of this power converter valve. This firing-circuit subassembly emits firing pulses that are determined from a synchronizing signal and a specific delay angle related thereto. A downstream logic unit also allows protective intervention to block firing in the event of malfunctions.
Each zero crossing of the synchronizing voltage generates the starting point of a ramp. The intersection points between the ramp and a straight line reproducing the delay angle define the valve triggering times for the positive and negative half-waves of the capacitor voltage. The downstream barrier logic unit makes it possible to block firing pulses for protective or startup purposes. A preparation circuit followed by signal amplifiers adapts the signals and their pulse lengths to the requirements of the valve electronics. The capacitor voltage of the controlled series compensator is used as the synchronizing voltage.
Since, in the ideal operating mode, the capacitor voltage is a non-sinusoidal variable with zero crossings that change in the course of the charge-reversal process, this variable is not suitable for stable synchronization of the firing-circuit subassembly. There is also the danger of a zero crossing shift due to superimposition of a DC voltage component on the capacitor voltage. If the capacitor voltage, with its changing zero crossings, is used as the synchronizing signal, stability problems will occur in the transient state when the controlled series compensator is being operated close to resonance. When the controlled series compensator is bypassed, the series capacitor is short-circuited, meaning that no further voltage is present at the capacitor. Therefore, in the bypassed state no synchronizing signal for the firing-circuit subassembly is present.
An underlying object of the present invention is to provide a method and an apparatus for generating a synchronizing signal for a firing-circuit subassembly to trigger a power converter valve of a controlled series compensator, without the aforementioned disadvantages.